1. Field of the Invention
The present invention relates to a process for controlling the operation of an electrolytic cell, for example, for controlling the electrolysis of aqueous solutions of sodium chloride (the only industrial process for producing chlorine and sodium hydroxide).
More especially, rather than employing, for example, a flow rate measurement for actuating a flow rate controller, and simultaneously a concentration measurement for actuating a temperature controller, all of the measurements are centralized according to this invention, such measurements being made coherent with the overall cell balance and appropriate signals being delivered to the various controllers.
2. Description of the Prior Art
Electrolysis is a process carried out industrially to produce, for example, alkali metal chlorates or alkali metal hydroxides. The electrolysis of sodium chloride solutions to produce chlorine and sodium hydroxide is the most important in terms of the final tonnages produced and because it is the only industrial-scale process employed today; see, for example, Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd edition, pages 799 to 865.
It is known to this art that control of the operation of a cell or group of electrolysis cells is generally effected by means of a servo system utilizing the parameter values supplied by characteristic sensors of the element(s) or compounds entering or exiting the installation. These values permit control over the operation of the installation, by virtue of control means to which a set point signal is supplied, together with signals corresponding to some of the parameters (for example the concentrations of residual compounds exiting the installation). These means of control supply a command signal which makes it possible, in particular, to issue commands to means for controlling the flow rates of the starting materials introduced into the apparatus.
Control systems of this type, which are well known to this art, incorporate at least one control loop and present disadvantages by reason of the fact that the values of the parameters supplied by the sensors are approximate values of these characteristic parameters and not highly accurate values. Consequently, a control device whose operation is based directly on the values of the characteristic parameters supplied by sensors does not permit an optimum control set point to enable an electrolysis cell to operate at an optimum efficiency.
The prior art proposes specific control systems for electrolysis cells. U.S. Pat. No. 4,035,268 describes a device for adjusting the separation of the electrodes in what is commonly designated a "mercury" cell process. European Patent EP No. 99,795 describes a system for controlling the current of a group of electrolysis cells. As above, these devices are only improved conventional controls, namely, those wherein a parameter has been analyzed and measured more precisely and then transmitted to a conventional controller.